The present invention concerns a process for preparing polyvinyl chloride (hereinafter simply referred to as PVC) material used for extrusion molding from a blend prepared by blending PVC material with plasticizers, stabilizers and other auxiliary materials and subjecting the blend to a dry blending step, a kneading step and a pelletization step successively.
The preparing process according to the present invention is a method suitable to the preparation of PVC material for extrusion molding products such as protector moldings and trims for use on automobiles, the appearance of which is considered important.
In the case of preparing the PVC material used for extrusion molding as described above, PVC (polymer) used so far has been synthesized by suspension polymerization and has a grain size corresponding to the retention rate on 80 mesh screen (ASTM; E11-58T) of about 5%.
In the dry blending step, a blend prepared by adding auxiliary materials such as plasticizers, stabilizers and colorants to PVC is subjected to blending under high shearing force at a temperature (80.degree.-110.degree. C.) much lower than the usual melting point (170.degree. C.) of PVC. The mixer used in this step is usually a high speed mixer such as a Henschel mixer or a super mixer.
In the kneading step and the pelletization step, the dry blend obtained as above (powdery product) has been kneaded in a single axis or a two-axis extruder, extruded as a strand of round or square cross section and then cut into an appropriate length by a cutter (rotary blade). The temperature in each of the kneading and pelletization steps is usually at 40.degree.-100.degree. C. on the material charging portion and at 170.degree.-200.degree. C. on the extrusion portion of an extruder.
In the case of extrusion molding at a high speed by using the PVC material produced as described above, irregularity (defective appearance) tends to occur at the surface of the extrusion molding product.
In this case, the surface irregularity as described above can substantially be eliminated from the extrusion molding product if the molding material is extruded after it has been kneaded and melted in a cylinder for a sufficient period of time, that is, if the extrusion speed is reduced. However, the reduction of the extrusion speed is not desired in view of the productivity.
In this case, the ratio of reducing the extrusion speed is within a range from 1 to 7 based on the speed for the conventional high speed extrusion being assumed as 10.